The department possesses two modern laserinterferometric measurement devices for particle velocimetry measurement (LDA, Dantec Corp.) and a combined particle velocity and size measurement (PDPA, Aerometrics/TSI Inc.). Both systems enable the determination of velocity (LDA, PDPA) and size of particles or small gas bubbles within a small volume in a flow and with high accuracy. The principle is based on the measurement of the Doppler shift of light that is scattered from the moving particles in a small volume of two crossing laser beams. Thereby the Doppler shift is measured from the temporal spacing of the interference fringes. By proper arrangement of multiple detectors the PDPA can additionally determine the size of the particles from the phase shift of the Doppler signals between the given spatial locations. Both techniques are inherently self-calibrating and are thus essentially independent from optical measurement conditions. The measurement is without intrusion into the flow and thus especially suitable in problems and places, where conventional probes are difficult to apply: reverible flows, chemically aggressive substances, high temperatures and pressure. Further, it is possible to perform measurements in moving systems, such as in reciprocating engines and turbo machines.
For more details see: DANTEC, TSI.
The LDA allows a one-dimensional and two-dimensional measurement of local velocity. A completion of the system for three-dimensional measurements is planned for the future. The LDA system comprises a SpectraPhysics argon laser, an optical Bragg modulator, two measurement probes, and a PC-controlled traversing unit for the probe(s). The Aerometrics PDPA has one probe for one-dimensional velocity and particle size measurements.